Drilling fluids, which are also called drilling muds, are complex mixtures of chemicals used in drilling operations for the production of hydrocarbons and natural gas from subterranean reservoirs.
Typically, oil and gas wells are drilled using drilling equipment in the presence of a drilling fluid.
Drilling fluids, generally comprising an emulsion or a solid suspension in a dispersing liquid phase, are pumped inside the drilling shaft and exit from the drilling bit through small openings.
The drilling fluids return to the surface through the small annulus between the outside of the drilling shaft and the bore hole wall.
Drilling muds perform a number of functions.
Exemplary of these functions are: cooling the drill bit; creating hydrostatic pressure to avoid uncontrolled blow outs and to help supporting the weight of the bore hole walls; acting as lubricant between the drilling bit and the bore hole walls; carrying drill cuttings up to the surface and suspending them when the fluid circulation is stopped.
Moreover, drilling fluids create a filter cake having low permeability on the bore hole walls and, in case, on the surface of the porous geological formations.
Drill cuttings may have the tendency to aggregate together in the drilling mud and to adhere on the drill string and casing; this phenomenon is often referred to as “accretion”.
The accretion of the drill cuttings to the drill string and the casing impairs drilling operations and in the worst case can even result in drill string and drill pipe sticking.
It is therefore of the utmost importance to find methods for preventing accretion.
Several treatments have been attempted to solve these problems, including addition of surfactants, inorganic salts or lubricants to the drilling mud, but none of these additives has proved to fully satisfy the expectations.
Examples of anti-accretion agents can be found in:                U.S. Pat. No. 5,605,879, disclosing the use of synthetic polyalphaolefins;        U.S. Pat. No. 5,639,715, describing additives based on sulfosuccinate chemistry;        U.S. Pat. No. 6,803,346, describing additives based on phosphonate chemicals;        U.S. Pat. No. 7,081,438, reporting the use of phosphonates or phosphate esters of organic ligands.        
A common problem which is encountered with the known anti-accretion agents is their possible toxicity to marine organisms and their poor biodegradability.